definitely nothing easy to deside and where to set the limits.
if you would be forbidden to use sencha based methods, then you would create pure js widgets and no sencha addons / ux.

it should also depend on how much (in %) sencha code you use. example: you change 2 lines of code in a big module and release it with a version that is not yet made public to non support subscribers. in that case there has to be at least a check if the buyer is the owner of a licence or the developers have to stick to the public versions.

i can not speak for sencha here, but i think there are a lot of points that should be thought through carefully.

This is a very valid question, and every add-on author will no doubt have the exact same question. There may be some situations that require contacting the licensing dept., but I'm sure that we can provide some general guidelines that most people will be able to follow. I'll make sure that the appropriate people see this question and see if we can get some guidelines put together.

Sometimes you can get this done with interceptor and/or sequence. It's lot of work to prevent overrides though.

In my experience, doing anything halfway serious always requires some overrides that include existing code. And even interceptor and sequence don't always allow you to avoid copying some existing code into your function (depends on where you have to inject your new logic). Speaking for myself, it would be impossible for me to ship my calendar components without some overrides that must, by nature, include some Sencha code. YMMV.